Pay Attention!!

Graph

directed graph or undirected graph?
The methods of graph traversal?
http://articles.leetcode.com/2012/05/clone-graph-part-i.html

Number
always consider if the algorithm will over flow!!!!!!
if negative or zero number will impact the final result!!!!!!
http://articles.leetcode.com/2012/01/palindrome-number.html

String
'a' to 'z'? Digits? Upper case letter? Does it contain ASCII characters only? Or even unicode character sets?
http://articles.leetcode.com/2011/05/longest-substring-without-repeating-characters.html

Tree
duplications? 
http://articles.leetcode.com/2011/04/construct-binary-tree-from-inorder-and-preorder-postorder-traversal.html